When optical waves of different wavelengths propagate along an optical fiber, they do so at different group velocities. This so-called chromatic, or group-velocity dispersion causes pulse broadening which limits the rate at which information can be carried through an optical fiber. Recently, there has been interest in dispersion compensators that use dual-mode fibers operated in the LP.sub.11 spatial mode, close to their cutoff wavelength. Because higher-order spatial modes in optical fibers exhibit large, negative chromatic dispersion, such an arrangement provides a means for compensating for the positive dispersion in conventional single-mode fiber spans. See, for example, U.S. Pat. No. 5,185,827, issued Feb. 9, 1993, and assigned to applicant's assignee.
The attractiveness of using a dual-mode fiber for dispersion compensation stems from the ability to achieve large negative waveguide dispersion by operating close to the cutoff wavelength of the LP.sub.11 mode, thereby minimizing the amount of fiber required to compensate a given amount of positive dispersion. While the use of less fiber reduces losses, as noted in the application of Craig D. Poole (Case 5), Ser. No. 08/150,513, filed Nov. 9, 1993, now U.S. Pat. No. 5,371,815 and assigned to applicant's assignee, operation close to cutoff greatly increases sensitivity to bending losses, and it is these losses that place a practical limit on how much dispersion compensation can be realized.
An alternative approach to this problem is the use of dispersion shifted fibers, as described in the article by V. A. Bhagavatula et at., entitled "Segmented Core Single-Mode Fibers with Low Loss and Low Dispersion," published in Electron Lett, 19, 317 (1983). However, such fibers are complicated structures and tend to be more lossy than one would like.
In U.S. Pat. No. 4,953,947, issued Sep. 4, 1990, a dispersion transformer is disclosed which employs a multimode delay optical waveguide to compensate for the dispersion produced in a transmission optical fiber. The multimode delay optical waveguide initiates the propagation of a plurality of modes which propagate therethrough at various velocities. The latter are selected to compensate for the dispersion introduced in the transmission fiber.
In U.S. Pat. No. 4,777,663, issued Oct. 11, 1988, a dispersion transformer is introduced into an optical system for the purpose of adding to the dispersion otherwise produced in an optical transmission system. The purpose in doing so is to limit the data rate of the system.
Both of the above-described systems employ multimode optical devices and fibers and, as such, are totally impractical as dispersion compensators. Any fiber-based device that is used to correct chromatic dispersion in a single-mode fiber must itself be single mode, where the term "single mode" is used in its accepted sense to mean that for a given wavelength, the fiber or device is supportive of only one propagating mode. This is so inasmuch as both the magnitude and functional form of chromatic dispersion in single mode fibers is different than that of any multimode device. In particular, the magnitude of modal dispersion is far greater than the chromatic dispersion in single mode fibers. In addition, the functional form of the dispersion depends upon the multimode device. In general, the functional form cannot be made the same as for a single mode fiber. Inasmuch as dispersion compensation must be made over a finite spectral width, this would preclude full dispersion compensation. A further, significant difficulty in using multimode devices for dispersion compensation in a single mode transmission system is that they would introduce excessive coupling losses. While light can be coupled from a single mode device to a multimode device without loss, the opposite is not true since coupling light into a single mode device requires that the input mode be matched to that of the single mode device. All the modes that are not matched are lost, resulting in large coupling losses.
It is, accordingly, an object of the present invention to effect low loss dispersion compensation in single mode transmission systems without the use of long lengths of optical fibers or specially designed fibers.